U.S. patent number 4,608,993 [Application Number 06/636,383] was granted by the patent office on 1986-09-02 for blood flow measurement device and method.
This patent grant is currently assigned to Quinton Instrument Company. Invention is credited to David E. Albert.
United States Patent |
4,608,993 |
Albert |
September 2, 1986 |
Blood flow measurement device and method
Abstract
The disclosure relates to systems for measuring blood flow by
detecting Doppler shift of ultrasound reflected by blood components
moving in a blood vessel. The systems employ electronic techniques
for providing accurate tracking of portions of the frequency
spectra of Doppler shift signals to determine peak and means
velocity and acceleration.
Inventors: |
Albert; David E. (McAlester,
OK) |
Assignee: |
Quinton Instrument Company
(Seattle, WA)
|
Family
ID: |
24551658 |
Appl.
No.: |
06/636,383 |
Filed: |
July 31, 1984 |
Current U.S.
Class: |
600/457;
73/861.25 |
Current CPC
Class: |
A61B
8/065 (20130101) |
Current International
Class: |
A61B
8/06 (20060101); A61B 010/00 () |
Field of
Search: |
;128/660,661,663,713,715
;73/861.25 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
Sainz et al, "A New Approach to Doppler Ultrasound Flowmetry",
Conference: Ultrasonics International, 1977, Brighton, England
(28-30, Jun. 1977) pp. 214-220. .
Callicot et al, "A Maximum Frequency Detector for Doppler Blood
Velocimeters", Journal of Medical Eng. and Technology, vol. 3, No.
2, Mar. 1979, pp. 80-82. .
Coghlan et al, "Improved Real-Time Spectrum Analyser for
Doppler-Shift Blood Velocity Waveforms", Medical & Biological
Engineering and Computing, May 1979, vol. 17, No. 3, pp. 316-322.
.
McCarty, "Frequency Modulated Ultrasonic Doppler Flowmeter",
Medical and Biological Engineering, vol. 13, No. 1, Jan. 1975, pp.
59-64. .
Gerzberg et al, "Power Spectrum Centroid Detection for Doppler
Systems Applications", Ultrasonic Imaging, vol. 2, No. 3, Jul.
1980, pp. 232-258. .
Skidmore et al, "Maximum Frequency Follower for the Processing of
Ultrasonic Doppler Shift Signals", Ultrasound in Med. & Biol.,
vol. 4, No. 2, 1978, pp. 145-147. .
Thomson, "Broadband Pulsed Doppler Ultrasonic System for the
Non-Invasive Measurement of Blood Velocity in Large Vessels",
Medical & Biol. Eng. & Computing, Mar. 1978, vol. 16, No.
2, pp. 135-146. .
Peeters et al, "A Self-Correcting, Phase-Locked Tracking Method for
Pulsed Ultrasound", IEEE Transactions on Biomedical Engineering,
vol. 26, No. 2, Feb. 1979, pp. 119-122..
|
Primary Examiner: Howell; Kyle L.
Assistant Examiner: Smith; Ruth S.
Attorney, Agent or Firm: Burns, Doane, Swecker &
Mathis
Claims
I claim:
1. An apparatus for measuring instantaneous peak velocity in blood
flow through a blood vessel, comprising
means for obtaining a Doppler signal having a frequency spectrum
determined by Doppler shifting of an ultrasonic signal caused by
relative movement of components of the blood in the blood vessel;
and
means for tracking a high frequency edge of said frequency spectrum
to produce a signal representative in value to the instantaneous
peak velocity of the blood components, said tracking means
including means for modulating said Doppler signal up to a
frequency above 100 kHz.
2. The apparatus of claim 1 wherein said Doppler signal obtaining
means comprises:
transducer means for directing ultrasonic energy at blood flowing
in a blood vessel and for receiving ultrasonic energy reflected by
blood components flowing in the blood vessel; and
means for electronically extracting audio frequency Doppler shift
signals from the reflected and received ultrasonic energy.
3. The apparatus of claim 2 wherein said modulating means
comprises:
controllable oscillator means for producing a signal having a
frequency greater than 100 kHz;
means for mixing the audio frequency Doppler shift signals with the
oscillator means signal to thereby modulate the Doppler signal up
to a frequency above 100 kHz;
wherein said tracking means further comprises a fixed, band pass
filter means for filtering the mixed signal; and
wherein the oscillator means is controlled so that a portion of the
mixed signal, corresponding to a high frequency edge of the
spectrum of the Doppler shift signal, is maintained in the pass
band of the filter.
4. The apparatus of claim 3 wherein the band pass filter means is a
ceramic filter with fixed band pass characteristics.
5. The apparatus of claim 3 wherein the band pass filter means has
a Q greater than 100.
6. The apparatus of claim 3 wherein the audio frequency Doppler
shift signals from the extracting means are band limited to between
about 100 Hz and 10,000 Hz.
7. The apparatus of claim 1 further comprising means for
differentiating said signal representative in value to the
instantaneous peak velocity of the blood components to produce a
signal representative in value to the instantaneous peak
acceleration of the blood flow.
8. The apparatus of claim 1 further comprising an automatic gain
control circuit for normalizing the power in the frequency spectrum
of said Doppler signal during the heart beat while preserving the
frequency distribution in the spectrum, including
a source of a sinusoidal signal having a frequency lower than about
1000 Hz; and
means for summing the Doppler signal and the sinusoidal signal and
for maintaining approximately constant the peak to peak amplitude
of the summed signal;
said gain controlled signal being applied to the tracking
means.
9. A method for measuring instantaneous peak acceleration in blood
flow through a blood vessel, comprising the steps of:
(a) directing ultrasonic energy at blood flowing in the blood
vessel;
(b) receiving ultrasonic energy reflected by blood components
flowing in the blood vessel;
(c) electronically extracting audio frequency Doppler shift signals
from the reflected and received ultrasonic energy;
(d) mixing the audio frequency Doppler shift signal with a signal
having a frequency greater than 100 kHz generated by a local
oscillator;
(e) filtering the mixed signal;
(f) controlling the local oscillator responsive to said filtered
signal to vary the frequency of the local oscillator signal so that
a portion of a side band of the mixed signal, corresponding to a
high frequency edge of the spectrum of the Doppler shift signal, is
maintained in a pass band of the filter; and
(g) differentiating with respect to time the filtered signal to
produce a signal representative in value to the instantaneous peak
acceleration of the blood flow.
10. The method of claim 9 further comprising the step of
calibrating the measurement of instantaneous peak acceleration in
blood flow by employing a band limited white noise signal with a
known cut-off frequency in place of said audio frequency Doppler
shift signal and adjusting the signal representative in value of
blood acceleration in accordance with a known calibration value
associated with said band limited white noise signal.
11. An apparatus for noninvasively measuring blood flow in a blood
vessel comprising:
ultrasonic probing means for obtaining a Doppler signal caused by
Doppler shifting due to reflection of ultrasonic energy from moving
components of the blood in the blood vessel including means for
demodulating the Doppler signal to produce an audio frequency
Doppler signal; and
means for receiving said audio frequency Doppler signal and for
producing a signal representative in value to the instantaneous
peak velocity of the blood components including modulation means
employing a local oscillator producing a signal of a frequency
greater than 100 kHz.
12. An apparatus for measuring blood flow through a blood vessel
comprising:
means for obtaining a Doppler signal produced by Doppler shifting
of an ultrasonic signal by reflection from moving blood in the
blood vessel;
an automatic gain control circuit for receiving said Dopper signal
and for normalizing the power in the frequency spectrum of said
Doppler signal during the heart beat while preserving the frequency
distribution in the spectrum, including means for summing the
Doppler signal with a sinusoidal signal having a frequency lower
than any Doppler frequency shift to be evaluated, and maintaining
the peak to peak amplitude of the summed signal at an approximately
constant value; and
means for tracking Doppler frequency shift in the Doppler signal as
it varies during the heartbeat and producing a signal
representative in value of a blood flow parameter.
13. The apparatus of claim 12 wherein said signal producing means
includes means for producing a signal representative in value of
instantaneous peak velocity.
14. The apparatus of claim 12 further comprising
means for differentiating the peak velocity signal with respect to
time to provide a signal representative of the instantaneous peak
acceleration of the blood.
15. The apparatus of claim 12 wherein said signal producing means
includes means for producing a signal representative in value of
instantaneous mean velocity of reflecting blood components.
16. The apparatus of claim 15 further comprising means for
differentiating the velocity signal with respect to time to provide
a signal representative in value of the mean acceleration of the
reflecting blood components.
17. A method for measuring blood flow through a blood vessel,
comprising the steps of:
obtaining a Doppler signal produced by Doppler shifting of an
ultrasonic signal by reflection from moving blood in the blood
vessel;
summing the Doppler signal with a continuous sinusoidal signal
having a frequency less than about 1000 Hz;
electronically maintaining said summed signal at an approximately
constant amplitude during the heart beat; and
electronically tracking frequency components of the approximately
constant amplitude signal to produce a signal representative in
value of the velocity of the blood.
18. The method of claim 17, wherein said tracking is performed by
heterodying the approximately constant amplitude signal with a
signal having a frequency above 100 kHz.
19. The method of claim 17, wherein the sinusoidal signal has a
frequency above the frequency of sounds produced by the heart
beat.
20. The method of claim 19, wherein the sinusoidal signal has a
frequency from 100 to 1000 Hz.
21. The method of claim 17, wherein the step of electronically
tracking frequency components include the determining of the
instantaneous mean velocity of the reflecting blood components by
applying the approximately constant amplitude signal to first and
second electronically controlled filters arranged in parallel, said
first filter passing relatively higher frequency components of the
Doppler signal and said second filter passing relatively lower
frequency components, said filters being controlled by a feedback
control signal so that the power in the portion of the Doppler
signals passed by each is maintained approximately equal, the
instantaneous mean velocity being determined from the feedback
control signal.
22. An apparatus for measuring the mean velocity of blood flowing
in a blood vessel comprising:
transducer means for producing ultrasonic energy directed at blood
flowing in the blood vessel;
means for receiving ultrasonic reflected by blood components
flowing in the blood vessel and for electronically extracting audio
frequency Doppler shift signals from the reflected and received
ultrasonic energy;
a first electronically controlled filter;
a second electronically controlled filter, wherein the Doppler
shift signal is applied to each of the electronically controlled
filters and wherein said filters selectively pass different
portions of the audio frequency spectrum of the Doppler shift
signal;
feedback circuit means for providing a signal for controlling the
first and second filter so that the portion of the Doppler shift
signal passed by each filter maintain a predetermined relationship
to one another, said feedback signal being related in value to the
instantaneous mean velocity of moving, reflecting components of the
blood in the blood vessel.
23. The apparatus of claim 23 wherein the said audio frequency
Doppler signal is band-limited and the spectrum of the band-limited
signal is divided into a high pass portion and a low pass portion
by the first and second filters, respectively.
24. The apparatus of claim 24 wherein the lower boundary frequency
of the high pass portion is substantially equal to the upper
boundary frequency of the low-pass portion.
25. The apparatus of claim 23 further comprising:
(a) circuit means for producing a signal responsive to an envelope
of an absolute value of signal peaks in the signal passed by the
first filter;
(b) circuit means for producing a signal responsive to an envelope
of an absolute value of signal peaks in the signal passed by the
second filter; and
wherein said feedback circuit means includes means for
differentially combining output signals of circuit means (a) and
(b), and for producing said feedback control signal therefrom.
26. The apparatus of claim 26 further comprising means for
differentiating the feedback control signal to obtain a signal
related in value to the instantaneous mean acceleration of the
reflection components of the blood.
27. The apparatus of claim 25 wherein the feedback circuit further
comprising a voltage controlled oscillator and wherein said filters
are monolithic and each are controlled responsive to a signal from
the voltage controlled oscillator, the control signal for said
oscillator being said feedback control signal.
Description
BACKGROUND OF INVENTION
1. Field of the Invention
The present invention concerns a blood velocity and acceleration
measurement system. More particularly, it concerns an improved,
non-invasive blood velocity and acceleration measurement device
based on the use of Doppler ultrasound, and capable of providing
measurements of peak velocity and peak acceleration of blood
components, as well as a mean velocity and acceleration thereof. In
addition, a method and apparatus for calibrating the blood flow
measurement devices are disclosed.
2. Brief Discussion of Prior Art
Velocity and acceleration of blood flow in blood vessels are
believed to be an important diagnostic tool. In particular, the
peak acceleration of aortic blood flow has been recognized to be an
excellent index of ventricular funcition. "A substantial body of
evidence indicates that the peak acceleration of the blood
corpuscles ejected by the left ventricle into the ascending aorta
is the most sensitive indicator of ventricular performance"
Rushmer, Cardiovascular Dynamics, p. 365 (4th ed. 1976). Rushmer's
conclusion is that the acceleration of blood corpuscles in the
aorta would be a valuable index of the influence on cardiac
function of various perturbations, e.g., such as coronary
occlusion, exercise, and drug infusion--a conclusion which Rushmer
demonstrated experimentally. Other observers have considered peak
acceleration, peak velocity, mean velocity, and mean acceleration
of the blood corpuscles to be useful indices of cardiac
performance.
There is a recognized need for a non-invasive indicator of
ventricular performance that can provide ready access to reliable
data regarding critical variables or parameters of cardiac
performance. ("Invasive" techniques are those involving physical
penetration of the body, such as by surgical opening of a portion
of the body to permit insertion of a measurement device, or by
injection of dyes to permit X-ray visualization.)
Recognized authorities have stated that it is not possible to
measure aortic blood flow acceleration non-invasively. See Gams,
Huntsman, and Chimoskey, Peak Aortic and Carotid Flow Acceleration
in Trained Unanesthetized Dogs, Federation Proceedings, American
Physiological Soc'y (1973). A prevalent technique presently used
for evaluating cardiac performance in human patients is the
radioisotope ventriculogram procedure. However, the necessary
equipment is extremely expensive; specifically trained and licensed
technicians are needed; and the tests involve insertion of a
catheter into the patient's vein. Other invasive techniques are
available in the case of animal experimentation, but the techniques
are usually unsatisfactory for human patients. For example, some
experimenters have inserted measuring devices into or adjacent to
the aortas of laboratory dogs and humans. In the past, such
invasive techniques have been used to strengthen medical
understanding of blood flow. Although they are helpful for research
purposes, such invasive techniques are usually not clinically
practical or create risks to the patient.
Doppler ultrasound is used as a clinical and research tool in the
evaluation of blood circulatory dynamics. The use of Doppler
ultrasound may involve determining the speed of a reflecting
material by beaming ultrasonic waves at the object and then
measuring the frequency shift in the ultrasound waves reflected by
the material.
Experimental work has indicated that ultrasound signals for blood
velocity measurement can conveniently be transmitted into the body
via the suprasternal notch, thereby facilitating evaluation of the
ascending aortic or distal aortic arch blood flow. This "acoustic
window" (as we may term the place via which ultrasound is beamed
into the body) may be used in many types of patients.
Presently available electronic circuitry for decoding Doppler shift
signals for blood flowmeter purpose generally use the
"zero-crossing detector" method of determining Doppler shift
frequencies. This is the method used, for example, in White U.S.
Pat. No. 4,205,687. The method develops an RMS value of frequency
which is not directly indicative of peak or mean velocity or
acceleration. As shown by Lunt in his paper Accuracy and
Limitations of the Ultrasonic Doppler Blood Velocimeter in
Ultrasound in Medicine & Biology, 2:1-10 (1975), it is hard to
achieve accurate blood velocity measurements with the zero-crossing
detector method.
Techniques commonly in use for detection of peak Doppler
frequencies present various problems. The fast Fourier transform
("FFT") technique requires expensive and elaborate equipment, and
presently known FFT systems are not fast enough to measure peak
acceleration. Other known peak frequency detection systems, such as
voltage-controlled high pass filters, phase-lock loop systems, and
double filters are noise sensitive, have a limited band width and
frequency response, and are sensitive to amplitude modulation
("AM") of the signal.
Skidmore and Follett, in Ultrasound in Med. & Biol., 4:145
(1978), suggest Doppler-shift measurement of blood velocity. They
suggest detection of maximum Doppler frequency by use of a voltage
controlled high pass filter. But their system has not been put into
commercial use. It is believed that the reason is that it is too
sensitive to noise. Also, it appears not to be able to measure the
frequencies associated with the highest velocity corpuscles, which
are considered of greatest interest.
Callicot and Lunt, in "A maximum frequency detector for Doppler
blood velocimeters", J. Med. Engineering & Technoloqy 3:80
(1979), note the use of phased lock loop techniques to measure peak
velocity and mean velocity, and disclose a system in which a
voltage controlled oscillator in a feedback loop is used to detect
maximum frequencies representative of blood corpuscle velocities.
Callicot et al does not show tracking of the high frequency edge of
the frequency spectrum. FIG. 2 of the article illustrates this in
that it shows that the system significantly underestimates the true
peak velocity as measured by the sonagram. The slope of the
velocity is not tracked. The system cannot respond rapidly enough
to accurately detect peak acceleration. Moreover the system cannot
measure peak velocity as shown by their illustrations. Neither
Skidmore and Follett's or Callicot and Lunt's circuit has
sufficient frequency response to accurately measure peak aortic
acceleration.
There, thus, exists a need for a non-invasive technique for
measurement of peak aortic acceleration. The need could be
satisfied by a Doppler ultrasound device, if one could be devised
(a) that was relatively inexpensive, noise-free, and insensitive to
AM, and (b) that had a sufficient bandwidth and frequency response
to register higher Doppler frequencies and rate of change of
frequencies. It is believed by the inventor that the need for a
non-invasive technique is well recognized in the art, and that
other workers have sought to satisfy this need by a Doppler type
device. The inventor does not believe that it is recognized in the
art that the above-stated requirements must be met for a
satisfactory Doppler ultrasound system. In any event, no such
satisfactory Doppler system known to the inventor is presently
available. The present invention concerns such a system: a
real-time Doppler ultrasound, non-invasive system for measuring
peak aortic (or other vessel) acceleration and velocity, which is
relatively inexpensive, noise free, insensitive to AM, and with
band width and frequency response sufficient to track high
velocity, high acceleration blood component movement. Moreover,
there is a need for non-invasive techniques for accurately and
inexpensively determining mean blood velocity and acceleration.
These needs are realized by the techniques disclosed as
follows.
BRIEF SUMMARY OF THE INVENTION
The invention described below is intended to provide an accurate
real-time Doppler ultrasound system for measuring peak and mean
velocity and acceleration in blood flow. The system may be employed
to track the leading edge of the Doppler spectrum. The leading edge
or highest-frequency part of the Doppler spectrum may be detected
by circuitry which includes an automatic gain control circuit and a
tracking circuit.
The tracking circuit may include a mixer for modulating the audio
frequency Doppler shift signal up to a frequency greater than 100
kHz. The modulated signal may then be applied to a high Q, fixed
frequency band pass filter. The filtered signal may then be used to
control a local oscillator which provides a carrier signal. The
carrier signal is mixed with the Doppler shift signal in the mixer.
The feedback signal which controls the local oscillator is
representative in value to the instantaneous peak velocity of
reflecting components in the blood flow.
Accordingly, a signal can be developed that is representative of
the highest frequency in the Doppler spectrum, and thus is
representative of peak velocity in blood flow. Differentiation of
the peak instantaneous velocity signal provides a signal related in
value to the peak instantaneous acceleration. The peak value of
this signal for a single heartbeat may be derived as an indication
of cardiac function.
Another implementation of the circuitry uses the difference between
the outputs of voltage controlled high- and low-pass filters to
vary the cut-off frequencies of the same filters, so that the
system tracks the mean-frequency of the Doppler shift signal rather
than the peak frequency.
An automatic gain control (AGC) circuit is provided for the Doppler
shift signal. The overall power in the spectrum of the Doppler
shift signal will be different at different times during a single
heartbeat. To improve the accuracy of the mean and peak velocity
and acceleration detection circuits, the overall instantaneous
power of the spectrum may be maintained approximately constant by
the AGC circuit. The gain control circuit allows the mean or peak
velocity tracking loops described above to follow more faithfully
frequency changes in the Doppler signal, by minimizing the effects
of amplitude change and inhibiting amplification of noise. In
preferred embodiments, the gain control circuit includes a local
oscillator for producing a low audio frequency sinusoidal signal,
for example at 440 HZ, which is summed with the Doppler shift
signal. The amplitude of the summed signal is maintained
approximately constant by a fast acting AGC circuit.
A calibration circuit is also provided for embodiments of the
present invention. The calibration circuit produces band-limited
white noise which simulates the instantaneous Doppler spectrum for
calibration purposes.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a vertical cross-sectional view of the aorta and left
ventricle of a human being, showing an ultrasonic transducer
apparatus placed on the suprasternal notch and directed toward the
ascending aorta.
FIG. 2 is a schematic block diagram of a front end portion of a
continuous wave, directional blood flow measurement circuit.
FIG. 3a is a three dimensional projection of Doppler shift spectra
showing the changes in the spectra with time corresponding to blood
velocity changes during a heart beat.
FIG. 3b is a graph of peak frequency shift during a heart beat,
plotted on the same time scale as FIG. 3a.
FIG. 3c is a graph of the first time derivative of the function
shown in FIG. 3b.
FIG. 3d is a graph of the frequency spectra of Doppler shift
signals at times d--d and e--e.
FIG. 4 is a schematic block diagram of a circuit for determining
peak velocity and peak acceleration from a Doppler shifted
ultrasonic signal.
FIGS. 5a-c are graphs of various signals appearing at point "A" in
FIG. 4.
FIGS. 6a & 6b and 7a & 7b are graphs of instantaneous
frequency spectra of Doppler shifted signals showing various filter
cut-off profiles.
FIG. 8 is a schematic circuit diagram of a gain control portion of
a blood flow measurement circuit.
FIG. 9 is a schematic circuit diagram of a filter and signal
processing portion of a blood flow measurement circuit for
determining peak velocity and peak acceleration from a Doppler
shifted ultrasonic signal.
FIG. 10 is a schematic block diagram of a circuit for determining
mean velocity and acceleration from a Doppler shifted ultrasonic
signal.
FIG. 11 is a graph of the spectrum of a Doppler shifted ultrasonic
signal showing low and high filter cut off profiles useful in the
circuit of FIG. 10.
FIG. 12 is a schematic block diagram for a circuit useful in
calibrating blood flow measurement devices.
DETAILED DESCRIPTION
The present invention relates to a non-invasive, ultrasound
hemodynamic monitor system. The achievement of highly accurate,
real time measurement of peak and mean blood flow velocity and
acceleration with a non-invasive probe permits the system to be
used in monitoring patients in clinical situations. It is
contemplated that the system may be used for the following more
specific purposes, among others: measuring blood perfusion to
specific vessels, ambulatory monitoring of patients, monitoring of
patients in standard treadmill exercise tests, monitoring by
paramedic or emergency mobile units (with or without telemetry);
anesthesiology monitoring in operating rooms; and evaluation of
cardiac function in patients receiving cardiotoxic
chemotherapy.
The non-invasive probing techniques for obtaining Doppler shift
signals are illustrated in FIG. 1. In FIG. 1, a portion of the
human body is indicated generally by the numeral 20. The chest of
the body is shown in cross-section as is the heart 22 and aorta 24.
Arrow B represents the flow vector of blood exiting the left
ventricle 26 through the aortic valve 28. It is the parameters of
this blood flow which are the subject of measurement. However, as
will be clear from the following, the techniques described herein
may be adapted to measuring blood flow parameters in other blood
vessels.
Non-invasive probing of blood flow in the aorta 24 may be achieved
by directing a beam of ultrasonic energy U into the aorta and
measuring the Doppler shift of reflections from moving blood
components in the aorta. Those blood components providing usable
reflection may be the red blood cells or erythrocytes. Ultrasonic
energy may be directed and returned through the suprasternal notch
30, which is an acoustic window of the body through which
ultrasonic energy will pass relatively unattenuated. A transducer
apparatus 32 may be employed to direct the ultrasonic energy into
the body and receive reflected ultrasonic energy.
It has been found (see, e.g., Papadofrangakis U.S. Pat. No.
4,265,126; and Aronson U.S. Pat. No. 4,103,679) that blood velocity
may be measured by the Doppler shift in accordance with a
relatively simple equation:
Where:
V=velocity of blood corpuscles;
S=velocity of ultrasound in tissue (1540 meters per sec);
D=Doppler frequency shift;
T=Transmitted ultrasound frequency;
a=angle between the blood velocity vector and the sonic vector from
the ultrasound transducer.
In their paper, Continuous Wave Ultrasonic Doppler for Measuring
Aortic Velocity and Acceleration (U. Wash., Center for
Bioengineering, 1977), Johnson, Fairbanks, and Huntsman indicate
that the mean value of angle a, when the suprasternal notch is used
as the acoustic window and the ascending aorta is the target, is
about 6.degree.. The cosine of 6.degree. is very close to 1.00.
Hence, the cos a term of Equation (1) is unimportant, whether or
not there are variations in angle, and the cosine term of the
equation therefore may be ignored. This obviates recourse to the
expedients discussed, for example, in Hassler U.S. Pat. No.
4,127,842. This also leads to a simplified version of Equation
(1):
FIG. 2 is a block diagram of a front end portion of a blood flow
measurement system constructed in accordance with the teachings of
the present invention. Transmitting and receiving transducers 50
may be provided and located in a hand held probe such as the device
32 shown in FIG. 1. A master oscillator 52 may provide an
ultrasonic signal (e.g., 2-10 MHz) which is amplified by amplifier
54 and applied to the transmitting transducer. Reflected ultrasonic
energy is detected by the receiving transducer and may be amplified
by amplifier 56.
The circuitry shown on the right side of line 58 in FIG. 2 is a
circuit for electronically extracting audio frequency Doppler shift
signals. The system employs a conventional quadrature detection
technique employing pairs of product detectors, low pass filters
and phase shift networks. An audio frequency Doppler shift signal
is produced at the output of the summing circuit 60. This signal is
applied to a band pass filter which band limits the Doppler shift
signal to a band ranging from about 100 Hz to about 10 kHz. In a
preferred embodiment, this band ranges from 300 Hz to 12,000 Hz.
The parameters of the band are dictated at the lower end by a need
to filter out low frequency sound produced by the heart itself and
by blood vessel vibration. At the upper end, the frequency cut off
is selected so that the band is wide enough to include the highest
detectable Doppler shifts encountered in the reflection of the
ultrasonic energy from the blood components.
The band limited audio frequency shift Doppler shift signal appears
at terminal 62 and is applied to further processing circuitry which
will be described below.
The signal appearing at terminal 62 will now be described in
connection with FIGS. 3a through 3d.
It will be apparent that blood velocity and acceleration varies
with time during a single heart beat. Accordingly, the Doppler
shift of the ultrasonic signals will vary with time during the
heartbeat. In addition, moving blood components from which
reflections are obtained will differ in velocity due to the fluid
dynamic properties of the blood, e.g., streamlining and boundary
layering. This results in a spectrum of Doppler shift frequencies,
at any given instant rather than a single spectral line. Doppler
shift spectra are graphed in three dimensions in FIG. 3a, to show
variations of power and frequency with time.
Several salient features of the plot of FIG. 3a will now be
discussed. The line 80 represents the leading or high frequency
edge of the Doppler shift signal. This line corresponds to the
highest frequency at any given point of time which is detectable
above the noise level of the system. The upwardly inclined portion
82 of line 80 represents the onset of systole. A lower line 84
represents the lower limit frequency cut-off of the signal at about
100 Hz-300 Hz.
FIG. 3b is a two-dimensional graph of peak Doppler shift frequency
with respect to time. It will be readily understood that the peak
frequency represented by line 80 is an analogue of the
instantaneous velocity of the fastest blood components as a
function of time. Point 85 represents the peak velocity achieved
during a single heart beat. The steep portion 86 of the curve 80
represents a rapid increase in the instantaneous peak frequency,
hence in the instantaneous peak velocity, of the blood during the
onset of systole. The curve is steepest at the point of peak
acceleration. The frequency value of the audio Doppler shift in the
region 86 may be approximately 1000 to 1500 Hz for a normal
heartbeat with the body at rest.
FIG. 3c is a plot representing the time differentiation of the
function of curve 80. It will be readily appreciated that this plot
represents the instantaneous peak acceleration of the blood
components as a function of time. The point of peak acceleration
during a single heartbeat is represented by the point 86. It should
be noted that point 86 is preceded and followed by extremely rapid
rates of change in the acceleration. This extremely rapid rate of
change is difficult to track and probably accounts for the failure
of the prior art to teach a workable system for detecting peak
velocity and acceleration. As used herein, the terms "velocity" and
"acceleration" are used in their generic sense without limitation
to the direction, i.e. acceleration is used synonymously with both
positive acceleration and deceleration; velocity is used to connote
speed both toward and away from the receiving transducer.
FIG. 3d is a plot of the power spectrum of the Doppler frequency
shifts occurring at an instant of time denominated by the line d--d
in FIG. 3a. It will be readily understood that the point 88
corresponds to the instantaneous mean velocity of the reflecting
blood components at the instant d--d. The line 90 reflects the
noise of the system. It has been observed emperically that the
frequency spectrum envelope 92 shown in FIG. 3d rolls off rapidly
at higher frequencies. This roll-off corresponds approximately, to
a 4th order low pass filter or approximately 40 dB per decade
roll-off. Accordingly, the magnitude of the noise level 90,
available with state of the art components, does not mask the
leading edge and, thus, the peak frequencies of the Doppler
spectrum.
Referring next to FIG. 4 a schematic block diagram is shown for a
circuit for determining peak velocity and peak acceleration from a
Doppler shifted ultrasonic signal. An input terminal 100 of the
circuit of FIG. 4 may be connected to the output terminal 62 of the
front end circuit shown in FIG. 2. That signal is applied to an AGC
circuit 102.
The AGC circuit 102 may include a sine wave generator 104 and a a
fast attack AGC circuit 106. The Doppler shifted audio signal
(discussed in connection with FIG. 3) is summed with a sinusoidal
signal from the sine wave generator at a summing circuit 108.
The frequency at which the sine wave generator 104 is operated, is
selected to be low enough so as not to obscure important
information bearing frequency bands in the Doppler shift signal.
The frequency may be selected to be less than 1000 Hz and in a
preferred embodiment, is selected as 440 Hz. The fast attack AGC
circuit 106 operates to maintain the peak to peak amplitude of the
summed signal at an approximately constant value.
The operation of the AGC circuit is best illustrated in connection
with FIGS. 5 which show various signals appearing at node A in the
circuit of FIG. 4. FIG. 5a illustrates the signal at node A when no
Doppler shift signal is being applied to terminal 100. This signal
is a replica of the sinusoidal signal from the sine wave generator.
In FIGS. 5b and c progressively larger Doppler shift signals are
applied at terminal 100 and summed at circuit 108. It should be
noted that the wave forms illustrated in Figures a, b and c have
substantially identical peak to peak values (e.g. 100 millivolts).
This effect is achieved by the fast attack AGC circuit 106.
The AGC circuit 102 operates to normalize the power in the Doppler
shift frequency spectrum while preserving the relative frequency
distribution within the spectrum.
The effect of the automatic gain control circuit may be best
illustrated with reference to FIG. 3d. FIG. 3d illustrates a plot
96 of power vs. frequency such as might occur at the instant of
time indicated as e--e in FIG. 3a. This signal would be applied to
terminal 100 of the automatic gain control circuit. As is apparent
from the figure, the total power in the spectrum 96 is
significantly less than the power in the spectrum 92. The automatic
gain control circuit tends to normalize the power in the various
spectra during a heartbeat. Thus, the power levels of spectrum 96
would be increased so that the spectrum 98 would be applied at node
A. However, the gain control function does not significantly
distort the frequency distribution or general shape of the Doppler
shift spectra.
The automatic gain control circuit also operates to prevent the
amplification of unwanted higher frequency noise components when
smaller amplitude signals are detected in the system. It will be
understood that when substantially no Doppler shift signal is
present at terminal 100, the wave form appearing at A will be
substantially as shown in FIG. 5a. This non-information bearing
sine wave signal will be filtered out in the subsequent
circuitry.
A circuit implementing the design of the automatic gain control
system illustrated in FIG. 4 is shown in FIG. 8, wherein like
components and features are identified with like numerals. The
circuit of FIG. 8 employs an XR2206 integrated circuit manufactured
by EXAR to generate a 440 Hz sine wave signal which is summed at
summing circuit 108. The fast attack AGC circuit is of conventional
design employing a feed back controlled field effect transistor
107.
With continued reference to FIG. 4, a frequency tracking circuit
110 of the present invention will now be discussed. The gain
controlled Doppler shifted audio signal at A is applied to a mixer
112 where it is mixed with a carrier signal produced by a local
oscillator 114. The local oscillator 114 is operated at a frequency
in excess of 100 kHz. The operation of the mixer is to modulate or
heterodyne the Doppler shift signal up to a much higher frequency
which is more readily filtered by a band pass filter. The frequency
of oscillation of the local oscillator 114 is controlled by a
feedback loop 116 so that the circuit tracks the leading or high
frequency edge of the Doppler shifted signal.
The signal appearing at the output terminal 118 of the mixer 112
may be the upper or sum side band of the two signals. The carrier
or local oscillator frequency and the lower side band may be
suppressed. The signal from the mixer is applied to a high Q band
pass filter such as a ceramic IF filter having a Q greater than
about 100. In a preferred embodiment of the present invention, the
filter employed is a ceramic filter manufactured by the Murata
Company designated as CFU455H. The filter has a center band pass
frequency of 455 kHz. In this system, the quiescent frequency of
the local oscillator 114 is likewise selected to be 455 kHz.
An output signal from the band pass filter 120 may then be applied
to an amplification circuit 122. The amplified output from the
amplifier 122 may then be applied to a halfwave rectifier and peak
follower circuit 124.
A circuit implementation of the frequency tracking circuit 110
shown in block form in FIG. 4 is illustrated in FIG. 9, wherein
like features and components are identified with like numerals. In
the circuit embodiment shown in FIG. 9, the Doppler spectrum at
node A is modulated up to 455 kHz in an interated circuit mixer
identified as MC1496L manufactured by Motorola Company. A nominal
455 kHz signal is generated by a voltage controlled oscillator
(VCO) 114 based on an integrated circuit identified as XR2207
manufactured by EXAR. The feed back loop 116 is designed to
maintain the energy overlap between the filter and the Doppler
spectrum constant by changing the VCO beat frequency. As the
sectral components of the Doppler shift signal increase in
frequency with the acceleration of blood, the feedback loop acts to
decrease the VCO frequency thereby lowering the spectrum of the
upper side band down from the 455 kHz pass band of the ceramic
filter 120. When no Doppler signal is applied at A, the VCO is
operated at a quiescent frequency of 455 kHz. As the spectrum
increases in frequency, the VCO compensates by decreasing the
frequency of its output signal, thereby keeping the energy integral
of the spectrum within the filter profile at an approximately
constant level. This effect is best illustrated with reference to
FIGS. 6 and 7. FIG. 6a illustrates a plot 200 of power vs.
frequency in a Doppler shifted signal such as would appear at A. As
blood velocity increases during the onset of systole, the Doppler
shift spectrum shifts to higher frequencies as illustrated by the
plot 202 in FIG. 6b. The dotted lines 204 illustrate the steep
filtering characteristics or profile of the circuit of the present
system.
The shaded areas in FIGS. 6 and 7 are proportional to the feedback
voltage. This voltage is also used as a measure of the frequency of
the leading edge. In the ideal case, the area would vary linearly
with the frequency of the leading edge on the X axis. It will be
apparent that the ideal case is better approximated by the
relatively steep slope of the filter characteristics in FIGS. 6a
and b than by the less steep slope of the filter characteristics
206 of the prior art shown in FIG. 7a and b. As the spectrum
increases in frequency, the VCO compensates by decreasing its
output frequency thereby keeping the energy integral of the
spectrum and filter at a fairly constant level. The steepness of
the leading edge of the filter characteristic minimizes the error
which might otherwise occur because of the fact that the higher the
peak frequency of the spectrum, the greater the overlap between the
spectrum and filter characteristic necessary to produce the
required error voltage. This difference is minimized and,
therefore, the linearity is improved with the steep filter. This
may be contrasted to the prior art systems such as that shown by
Calicott and Lunt which employ an audio frequency VCO signal and
filters with necessarily lower Qs.
The tracking circuit complements the operation of the AGC circuit,
in that the 440 Hz signal summed at circuit 108 sets the maximum
gain of the AGC circuit so that when the Doppler signal amplitude
is very low, the AGC gain does not increase to the maximum and
thereby amplify noise. The signal emerging from the AGC circuit at
A is such that the peak velocity tracking loop need track only
frequency changes and not amplitude changes in the Doppler
spectrum.
With continued reference to FIG. 9, the halfwave rectifier-peak
follower circuit 124 may include a differential amplifier 126 and a
feedback loop including Shottkey diodes 128 which perform a
half-way rectification function. The signal appearing at node 130
is employed as a feedback signal for controlling the VCO 114 and
also as a signal representative in value of the leading edge of the
Doppler shift frequency spectrum. The loop circuit of the frequency
tracking circuit 110 has an intrinsic gain which can be expressed
numerically in terms of amplification divided by Hertz. In other
words, the signal emerging from the peak follower at node 116 has a
dc voltage value. For each increase in that dc voltage there is a
corresponding increase in units of frequency in the output of the
VCO 114. This quantity may be referred to as the "tracking system
gain function." It has been found that a desirable value of the
tracking system gain function for the preferred bandpass ceramic
filter device is 12.5 kHz per volt DC. Higher values of tracking
system gain function tend to lead to instability. Lower values of
the tracking system gain function do not optimize the fidelity of
tracking achievable with the circuit disclosed.
With continued reference to FIGS. 4 and 9, further signal
processing circuitry will be described. The tracking signal
appearing at node 130 may be applied to level adjust circuitry 140
employed to calibrate the system. The level adjusted signal is then
applied to two clock controlled filters 142 and 144 whose filter
characteristics are dictated by a clock circuit 146. The filters
142 and 144 are adjustable low-pass filters which smooth the output
of the peak velocity tracking circuit without removing important
velocity and acceleration information. Clock noise introduced into
the signal may be removed by a clock noise filter 148.
The output signal of the clock noise filter 148, appearing at node
150 is representative in value of the peak Doppler frequency
component. This signal may be differentiated by an operational
amplifier differentiator 152. The differentiated signal, appearing
at node 154 is representative of peak instantaneous
acceleration.
Both the peak instantaneous velocity and peak instantaneous
acceleration signals may be fed to peak detector circuits [not
shown] which detect and hold the peak values of the signals
observed at nodes 156 and 158, respectively. A calibrated visual
display may be provided that directly shows peak velocity and peak
acceleration in convenient units in real time such as in meters per
second and meters per second.sup.2. Alternatively, the successive
peak velocity and acceleration measurements can be recorded for
later reference.
Another embodiment of the present invention is illustrated in FIG.
10. A gain controlled Doppler shift signal from node A of FIG. 4
may be applied to the input terminal 300 of the circuit shown in
block diagrammatic form in FIG. 10. The circuit of FIG. 10 operates
on this signal to provide mean blood velocity and mean acceleration
measurements. The circuit of FIG. 10 employs a parallel pair of
frequency controlled filters; one, a high pass filter 302, the
other, a low pass filter 304. In a preferred embodiment, both
filters are operated so as to have the same cut off frequency. In
the preferred embodiment the voltage controlled high pass and low
pass filters may be built around a monolithic capacitor filter
device known by the manufacturers designation MF10, manufactured by
National Semiconductor, Inc.
The high pass filter 302 should, advantageously, roll off steeply
at approximately the maximum Doppler frequency anticipated [12 khz]
in order to avoid introducing error by integrating noise above that
frequency. An absolute value of the signal passed by the high and
low pass filters may be obtained by the absolute value circuits 306
and 308, which may be constructed in the same manner as the
absolute value circuits described in connection with FIG. 9. Output
signals of the absolute value circuits may then be filtered by low
pass filters 310 and 312 respectively.
A difference in signal is produced by the differential amplifier
314. An output difference signal from the differential amplifier
314 is used to control a voltage controlled oscillator 116. A
variable frequency signal from the voltage control oscillator is
returned via a feedback loop to control the cutoff frequencies of
the two voltage controlled filters 302 and 304.
The operation of the feedback loop is best described in connection
with FIG. 11. In FIG. 11 the envelope of the frequency spectrum 350
of a Doppler shifted signal is plotted. Lines 352 represents the
cut-off frequency profile of the voltage controlled low pass filter
304. Similarly, lines 354 represent the cut off frequency profile
of the voltage controlled high pass filter 302. The feedback loop
in FIG. 10 including the voltage controlled oscillator 316 operates
so that the area under the curve 350 and within the low pass filter
profile 352 is approximately equal to the area under curve 350 and
within the high pass filter profile 354. It will be appreciated
that the cut off frequency f.sub.c of the subsystem will move back
and forth along the X axis of FIG. 11 as the Doppler shift signal
varies during the heartbeat. It will also be appreciated that the
cut off frequency f.sub.c corresponds approximately to the mean
shifted frequency in the Doppler shifted signal. This in turn
corresponds to the mean velocity of the reflecting blood components
measured by the system.
The signal produced at node 318 is representative in value of the
instantaneous mean velocity of the reflective components of blood.
This signal may, in turn, be time differentiated by the
differentiator circuit 320 to produce a signal at node 322 which is
representative in value of the instantaneous mean acceleration of
the blood. Finally, peak detectors 324 and 326 may be employed
which detect the peak mean acceleration and peak mean velocity of
the blood during a single heartbeat. As with the previous
embodiments these signals may be applied to suitable recording or
displaying apparatus.
FIG. 12 is a schematic block diagram of a calibration circuit
suitable for use with embodiments of the present invention
discussed above. The calibration circuit consists of a white noise
source 400, the energy output of which may be approximately flat
from D.C. to 20 KHz. This output signal is fed to a voltage
controlled low pass filter 402, the cutoff frequency of which can
be varied from 100 Hz to 20 KHz. The filter cutoff frequency can be
controlled by a time-varying voltage source 404 or a calibrated
variable control voltage 406. The selection of control voltages may
be accomplished by means of a mechanical switch 408. A filtered
signal from the filter 402 may be applied to a variable gain
amplifier 410. An output signal at terminal 412 of the amplifier
410 may be applied as a calibration signal to the blood velocity
and acceleration measurement circuits described above.
The output signal of the calibration circuit is a band limited
white noise signal which simulates the instantaneous Doppler
spectrum produced by blood motion in the aorta. A known spectrum of
constant amplitude and frequency distribution is produced when the
calibrated control voltage is used. In contrast, varying spectra
are produced by the circuit when it is controlled by the
time-varying voltage. Such spectra simulate the time varying
spectra characteristic of heart beats.
The measurement of blood motion can be calibrated by employing the
band limited white noise signal with a known cutoff frequency from
the calibration circuit in place of said audio frequency Doppler
shift signal and adjusting the system output signal (normally
representative of a blood flow parameter) in accordance with a
known calibration value associated with the band limited white
noise signal.
Although the invention has been described in connection with
preferred embodiments, it is to be understood that variations and
modifications may be resorted to as will be apparent to those
skilled in the art. Such variations and modifications are to be
considered within the purview and the scope of the claims appended
hereto.
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